The invention relates to a method of manufacturing an elongated product, comprising forming a core having an outer surface of plastic, extruding a seamless metal layer on the core allowing a clearance between the metal layer and the core, cooling the metal layer and reducing the diameter of the metal layer to get it in contact with the plastic surface of the core.
The invention further relates to an apparatus for making an elongated product, the apparatus comprising a plastic extruder for extruding a core, a metal extrusion machine for extruding a seamless metal layer outside of the core, the metal layer having an inner diameter larger than the outer diameter of the core so that the metal layer does not contact the core, and reducing means for reducing the diameter of the metal layer to get it in contact with the plastic core.
The invention further relates to an elongated product comprising a core having an outer surface of plastic and a seamless tubular metal layer extruded and drawn down on the core.
Multilayer composite pipes which have an inner and an outer layer of plastic and an aluminium layer between the inner and outer layers are well known. Such pipes are made for example such that the inner layer is extruded and coated with tie material. Thereafter, an aluminium band is wrapped around the inner layer and welded such that a longitudinal welding seam is formed. The welded aluminium layer is calibrated and the tie material is activated for bonding the inner layer with the aluminium layer. Thereafter, the aluminium layer is coated with tie material and an outer plastic layer is extruded on the aluminium layer. Such a solution is disclosed for example in EP 0691193. It is also possible to make the pipe such that first the aluminium band is wrapped to form a pipe such that the edges of the aluminium band overlap. Thereafter, the overlapped areas are longitudinally welded with ultrasonic welding. It is also possible to wrap the band such that the edges do not overlap and use butt-welding. Thereafter, the formed aluminium pipe is coated from the inside with tie material and plastic material forming the inner layer, and the outside of the aluminium layer is coated with tie material and plastic that forms the outer layer. In both technologies, it is very difficult to make the welding seam in a reliable way and such that the quality of the welding seam is even. Irregularities in the welding seam could lead to breaks of pipes and the welding seam quite easily breaks during expanding of the pipe end.
DE 2139388 discloses making of a pipe that has an inner layer made of plastic. Seamless metal layer, for example of aluminium, is pressed on the outside of the plastic layer. The aluminium is pressed directly on the plastic core. The temperature of the pressed aluminium is so high that it easily melts and damages the plastic core.
EP 0125788 also discloses extruding a seamless metal layer outside a plastic core. A mandrel is provided with an internal cooling shroud to protect the core from the hot pressed metal. The metal is extruded with an internal diameter greater than the external diameter of the core to permit the intervention of a portion of the cooling shroud and subjected to a stream of cooling air. To eliminate the space between the core and the pressed metal so that the core is tightly clad in a tubular sheath, it is necessary that the extrusion stage be followed by a step in which the metal tube is drawn or swagged down. However, this step work-hardens the cladding, making the product difficult to manipulate. Thus, the hardness of the product increases and the product becomes stiffer.
U.S. Pat. No. 5,222,284 discloses making a coaxial cable. An elongated core consisting of a conductor coated with an insulator is continuously compacted to reduce the cross-section of the core. A tubular metal cladding is continuously extruded outside of the elongated core and simultaneously the compacted core is continuously fed into the cladding, whereby the compacted core recovers towards its original cross-section to fill the cladding. Thus the core does not touch the metal cladding while the metal is still hot and therefore the damaging of the insulator can be avoided. Further, because the diameter of the metal cladding is not reduced, the hardening of the metal is avoided. However, the outer layer of the core must be made from an insulating material that can be compacted to reduce its cross-section by the application of compressive force. Further, the insulating material must be such that it gradually recovers such that the core tends to return to the original dimensions when the compressive force is relieved. The solution is rather complicated. Further, it is rather difficult to ensure the adhesion between the core and the metal cladding.